1. Field of the Invention
The invention relates to a method of producing a rotor of a motor.
2. Description of Related Art
In the automobile industry, development of higher-powered, lighter and more compact driving motors has been constantly advanced for the purpose of further improving travelling performance of hybrid vehicles and electric vehicles. Similarly, household electric appliance manufacturers have focused on development of more compact and higher-powered motors incorporated in various kinds of household electric appliances.
How to reduce various kinds of losses caused inside motors is an important issue to be addressed in order to improve performance of the motors. For example, after application of electricity to a motor, a copper loss due to a conductor resistance loss is caused in coils of the motor, and an iron loss (or a high-frequency iron loss) due to an eddy-current loss or a hysteresis loss is caused in a rotor or a stator of the motor. These losses lower the motor efficiency and the torque performance. The eddy-current loss is a loss caused by a change in the magnetic flux density, and the hysteresis loss is a loss due to a magnetic flux density waveform.
Examples of a method for reducing the eddy-current loss include a method of producing a rotor by inserting permanent magnets (rare-earth magnets) each constituted by a plurality of split magnets into magnet slots of the rotor. Methods of producing such split magnets are described in Japanese Patent Application Publication No. 2003-134750 (JP 2003-134750 A), Japanese Patent Application Publication No. 2-250921 (JP 2-250921 A), and Japanese Patent Application Publication No. 2-250922 (JP 2-250922 A).
The method of producing a permanent magnet described in JP 2003-134750 A includes a first step of splitting a permanent magnet base material into a plurality of magnet pieces, a second step of performing an insulation coating process on the entirety of all the surfaces of each of the magnet pieces obtained by splitting the permanent magnet base material, a third step of joining together the magnet pieces that have undergone the insulation coating process, a fourth step of machining a joined body such that the joined body has predetermined dimensions, and a fifth step of performing an insulation coating process on the entirety of all the surfaces of the joined body after machining.
According to the method of producing a rare-earth magnet described in JP 2-250921 A, a plurality of alloy materials for a rare-earth magnet is sealed in a metal capsule with a lubricant, and hot forging is performed while the metal capsule is restrained in the width direction. According to the method of producing a rare-earth magnet described in JP 2-250922 A, hot rolling is performed in the same manner as that described in JP 2-250921 A.
With the production method described in JP 2003-134750 A, it is possible to produce a permanent magnet configured to more reliably reduce generation of an eddy current. However, this production method requires many production steps, such as the step of performing an insulation coating process on the entirety of all the surfaces of each of the magnet pieces, and the step of joining together the magnet pieces that have undergone the insulation coating process. This makes the production of a permanent magnet considerably cumbersome and complicated.
Further, an integrated magnet constituted by a plurality of magnet pieces produced individually as described above may fail to be accurately inserted into and installed in a magnet slot of a rotor, if the dimensional accuracy of each of the magnet pieces is not rigorously controlled. This is because the dimensional deviation of the whole integrated magnet is the sum of the dimensional deviations of all the magnet pieces.
With the production methods described in JP 2-250921 A and JP 2-250922 A, it is possible to stably produce a rare-earth magnet having a good magnetic property. However, there is a need to prepare a metal capsule for each of the rare-earth magnets to be produced, and hot forging or hot rolling is performed after a plurality of alloy materials is sealed in the metal capsule. Thus, these production methods may require a lot of time and effort.